Frequency discriminator-power divider



March 22, 1955 c, w, ZABEL 2,704,810

FREQUENCY DISCRIMINATORPOWER DIVIDER Filed April 3, 1946 INVENTOR F|G 2 CARROLL w. ZABEL- ATTORNEY United States Patent FREQUENCY DISCRIMINATOR-POWER DIVIDER Carroll W. Zabel, Cambridge, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application April 3, 1946, Serial No. 659,189

12 Claims. (Cl. 250-31) This invention relates to hollow wave guide junctions and more particularly to such junctions for microwave frequency discrimination.

There are numerous applications for a frequency discriminator in the microwave frequency region. One of the most common is automatic frequency control for microwave oscillator stabilization. Heretofore in this application the microwave oscillator output has been beat against a microwave local oscillator to produce an intermediate frequency. This intermediate frequency in turn was then used in the well known Seely-Foster discriminator and the output thereof used to regulate the microwave oscillator frequency. This method of automatic frequency control requires numerous components and is diificult to adjust and maintain in adjustment.

Hollow wave guide junctions have previously been used to divide microwave power and in particular such a junction has been devised to provide adjustable microwave power division.

Therefore it is the object of this invention to provide a modification for previous hollow wave guide junction power dividers which will enable them to be used in frequency discriminator applications.

This and other objects will become apparent upon consideration of the following description in conjunction with the accompanying drawings in which:

Fig. 1 is a perspective viewof a hollow wave guide junction incorporating one embodiment of this invention; and

Fig. 2 is a group of curves showing the characteristics of this invention in a frequency discriminator application.

Fig. 1 discloses a rectangular wave guide junction hereinafter known as turnstile surmounted by a length of circular wave guide 11. The four arms of turnstile 10 lie in a plane perpendicular to the axis of circular wave guide 11 and are at right angles to one another. Power is fed into turnstile 10 through arm 12 and crystal detectors 23 and 24 are mounted in arms -13 and 14 adjacent to arm 12. Matching stub 15 is mounted at the junction of the arms of turnstile 10. A plurality of pins 16 are mounted in a single plane hereinafter called the plane of symmetry to form an effective quarter wave length plate, on the inside wall of circular wave guide 11 near the end thereof adjacent to turnstile 10. The pins are so mounted that their axes are perpendicular to the axis of the circular wave guide 11 and their plane of symmetry is at 45 degrees to the plane containing opposite arms of turnstile 10 and the axis of circular wave guide 11. The section of circular wave guide 11 farthest from turnstile 10 forms a resonant cavity 17 of adjustable dimensions. One end of resonant cavity 17 is a plug 18 mounted in a fixed position in circular wave guide 11. An aperture 19 in plug 18 couples power from the section of circular Wave guide 11 containing the effective quarter wave plate 16 into resonant cavity 17. The other end of resonant cavity 17 is a shorting plunger 20 which is adjustable in position longitudinally along circular wave guide 11 by means of micrometer screw 21.

Turnstile 10 has the following properties: microwave power fed into turnstile 10 through arm 12 divides so that one quarter is coupled into each of the opposite arms 13 and 14, the power in the two being in phase, none is coupled into the opposite arm 22, and one half is coupled into circular wave guide 11 with the electric "ice.

field vector being parallel to the direction of the incident wave in arm 12. Furthermore power fed into turnstile 10 through circular wave guide 11 with its electric field vector parallel to the axis of the two opposite arms 13 and 14 divides between these arms so that one half couples into each arm, the power in the two being 180 degrees out of phase, and none is coupled into power input. arm 12 or the opposite arm 22. Matching stub 15 is given a diameter and is so positioned with respect to its depth of insertion at the junction that it represents a matched termination for all of the wave guide arms and no power is reflected into power input arm 12 or circular wave guide 11.

The section of circular wave guide 11 containing the effective quarter wave plate formed by pins 16 has the following properties: a wave whose electric field vector is parallel to the plane of symmetry of a quarter wave plate will be shifted degrees in the time phase with respect to a wave whose electric field vector is perpendicular to the plane of symmetry in passing by the quarter wave plate. The pins 16 which constitute the effective quarter wave plate are so dimensioned and so spaced as to provide an impedance match for power coupled into circular wave guide 11 and at the same time to accomplish exactly a 90 degree shift in time phase as mentioned above.

Resonant cavity 17 operates in the TE111 mode and its resonant frequency is adjustable by movement of shorting plunger 20 by means of micrometer screw 21. Resonant cavity 17 is excited by means of aperture 19 which is so dimensioned that when the frequency of the power in circular wave guide 11 is equal to the resonant frequency of resonant cavity 17, an impedance is presented at plug 18 equal to the characteristic impedance of circular wave guide 11. This means that no power will be reflected by plug 18. However, when the frequency of the power in circular wave guide 11 varies even slightly from the resonant frequency of the resonant cavity 17, an impedance is presented at plug 18 which varies from the characteristic impedance and varying amounts of power will be reflected by plug 18 with varying shifts in time phase. Conversely, if the frequency of the power in circular wave guide 11 is maintained constant and the resonant frequency of the resonant cavity 17 is varied, the same variations in reflection at plug 18 will occur. This variation in reflection is equivalent to substituting a shorting plunger for plug 18 and moving it longitudinally along circular wave guide 11 but it is accomplished by a relatively small movement of shorting plunger 20 in changing the resonant frequency of resonant cavity 17.

This device is made use of as a frequency discriminator by inserting power into turnstile 10 through arm 12- and comparing the outputs of the crystal detectors in opposite arms 13 and 14. Fig. 2 shows the output of these crystal detectors for various input frequencies above and below the resonant frequency can of resonant cavity 17. Curve A represents the output from the crystal detector in arm 13, curve B that from the crystal detector in arm 14, and curve C represents the difference between curves A and B. Thus it will be seen that at the resonant frequency of resonant cavity 17 the output from the crystal detector in arm 13 minus that from the detector in arm 14 will be zero. However, below this resonant frequency the difference will be negative and above it will be positive. Thus in the automatic frequency control application, for instance, a negative voltage is available for one type of correction for the microwave oscillator and a positive voltage is available for the other type. The reasons for this phenomenon are apparent when the characteristics of the various parts of the device are recalled. Power fed into turnstile 10 through arm 12 divides, so that one quarter is coupled into each of opposite arms 13 and 14 and this power in these arms is in phase. The other half of the power is coupled into circular wave guide 11 with the electric field vectors being parallel to the direction of the incident wave in arm 12. Thus a wave is started past the quarter wave plate 16 toward resonant cavity 17 whose electric field vector is at 45 degrees with respect to the plane of symmetry of quarter wave plate 16. This wave can be thought of as two components, one having an electric field vector perpendicular and the other an electric field vector parallel to the plane of symmetry. The characteristics of the effective quarter wave plate in are such that the latter component will be shifted 90 degrees in time phase with respect to the former in passing along the length of the effective quarter wave plate 16. Therefore, beyond the effective quarter wave plate 16 there'is a wave moving toward resonant cavity 17 which comprises two components having electric field vectors at right angles to each other and 90 degrees out of time phase. This situation is comparable to the rotating magnetic field produced by the stator winding of a two phase induction motor. In the present case there is a wave traveling along circular wave guide 11 with its electric field vector rotating. If the frequency of the power fed into turnstile 19 through arm 12 is equal to the resonant frequency of resonant cavity 17, nothing will be reflected at plug 18 and the power into each of opposite arms 12* and 14 will be one quarter of the total and the outputs of the crystals in these arms will be equal. However, if the power is not equal to the resonant frequency, more or less of the wave traveling toward resonant cavity 17 will be reflected at plug 18. in passing along the length of quarter wave plate is? the reflected wave will have its component with the electric field vector parallel to the plane of symmetry of quarter wave plate 16 again shifted 90 degrees in time phase. Therefore the wave after passing along the length of quarter wave plate 16 toward turnstile it! will comprise two components, one having its electric field perpendicular to and the other parallel to the plane of symmetry. However, the latter component will be 180 degrees out of time phase with respect to the corresponding component of the previously mentioned wave traveling in the opposite direction at this point. The resultant wave is one having its electric field vector parallel to the axis of the two opposite arms 13 and 14. The characteristics of turnstile are such that this type of wave divides between the opposite arms 13 and 14 so that one half couples into each arm but the power in the two is 180 degrees out of phase. Thus this power will add to the one quarter of the total power coupled into one arm directly from arm 12 and subtract from this power in the'other arm. The magnitude of the power to be added or subtracted and the particular adjacent arm in which it will be added and not subtracted and vice versa depends upon the types of reflection that took place at plug 18 which, in turn, depends upon whether the frequency of the power fed into turnstile 1! was above or below the resonant frequency of resonant cavity 17. The overall effect, as mentioned above, is evident in Fi 2.

this device can also be made use of as an adjustable power divider by inserting power of a constant frequency into turnstile ill through arm 12 and varying the resonant frequency of resonant cavity 17 by means of micrometer screw 21. At a resonant frequency equal to the frequency of the input power nothing will be reflected at plug 18 and there will be an equal amount of power in each of opposite arms 13 and 14 respectively the magnitude thereof being one quarter of the total input power. As the resonant frequency of resonant cavity 17 is varied on either side of the frequency of the input power, however, the ratio of the powers in opposite arms 13 and 14 will change.

For a more detailed description of the characteristics of the turnstile 10 and the quarter wave plate 16 in circular wave guide 11 reference is made to a copending application of Robert H. Dicke, Serial No. 626,848, filed November 5, 1945, now Patent No. 2,686,901, dated August 17, 1954.

Reference is also made to. the Radiation Laboratory Series published by McGraw-Hill Book Company, Ltd., 1948, volume 8, by Montgomery et al., Principle of Microwave Circuits, and in particular to chapter 12, pages 459 to 466 concerning the turnstile junction; and to volume 14, by Smullin et al. on Microwave Duplexes, with particular reference to chapter 8, section 8.14, pages 372 to 375 concerning turnstile duplexes.

This invention is to be limited in no way by the foregoing description but only by the appended claims.

What is claimed is:

1. A hollow waveguide junction comprising, a main circular waveguide arm, four rectangular waveguide arms, said rectangular waveguide arms being parallel connected and symmetrically joined with said circular waveguide arm, the axes of said five waveguide arms being mutually perpendicular and meeting in a common point, a quarter wave length phase shifter positioned in said circular waveguide arm for rotating the polarization of radiation in said circular arm through 90 degrees, said circular wave guide arm being terminated in a cavity resonator for reflecting the radiation in said circular arm, the energy incident in one of said rectangular waveguide arms being divided between the adjacent rectangular waveguide arms.

2. A hollow waveguide junction comprising, a main circular waveguide arm, four rectangular waveguide arms, said rectangular waveguide arms being parallel connected and symmetrically joined with said circular waveguide arm, the axes of said five waveguide arms being mutually perpendicular and meeting in a common point, a quarter wave length phase shifter positioned in said circular waveguide armfor rotating the polarization of radiation in said circular arm through 90 degrees,

and said circular waveguide arm being terminated in a cavity resonator for reflecting the radiation in said circular arm, the fraction of said radiation which is reflected being a function of the difference between the frequency of said radiation and the resonant frequency of said cavity, the energy incident in one of said rectangular waveguide arms being divided between the two opposite rectangular waveguide arms adjacent said one waveguide arm, the ratio of energy division between said two waveguide arms being a function of said frequency difference.

3. Apparatus for frequency discrimination at high frequencies comprising, the hollow wave guide junction of claim 2, and detecting means in said opposite two rectangular waveguide arms, the difference voltage output of said detectors being a function of the frequency of said incident radiation relative to the resonant frequency of said cavity.

4. Apparatus for frequency discrimination at high frequencies comprising, a main circular waveguide arm, four rectangular waveguide arms, said rectangular waveguide arms being parallel connected and symmetrically joined with said circular waveguide arm, the axes of said five waveguide arms being mutually perpendicular and meeting in a common point, means for rotating the polarization of radiation in said circular waveguide through degrees, means including a tunable resonant cavity for reflecting radiation in said circular waveguide arm, the fraction of said radiation which is reflected being a function of the difference between the frequency of said radiation and the resonant frequency of said cavity, said circular arm, and an opposing pair of said rectangular arms having primary energy waves excited therein by radiation incident in one of the adjacent said rectangular arms, said opposing pair of rectangular arms having secondary energy waves excited therein by said reflected and rotated radiation in said circular arm, the time phase and magnitude of said secondary waves being functions of the frequency of said incident radiation relative to the resonant frequency of said cavity, the resulting energy waves in said opposing pair of rectangular arms being the vector sum of said primary and secondary energy waves, a first crystal detector placed in the first of said opposing pair of rectangular arms, and a second crystal detector placed in the second of said opposing pair of rectangular arms, the difference voltage output of said first and second crystal detectors being a function of the frequency of said incident radiation relative to the resonant frequency of said cavity.

5. Apparatus for frequency discrimination at high frequencies comprising, a hollow wave guide junction formed by acircular wave guide arm and four rectangular wave guide branches, said rectangular wave guide branches being parallel connected and symmetrically joined with said circular wave guide arm, the axes of said five wave guide branches meeting in a common point and being mutually perpendicular, a tunable cavity resonator coupled to the extremity of said circular wave guide arm, the proportion of energy reflected from said resonator being proportional to the difference between the frequency of energy in said arm and the resonant frequency of said resonator, and a quarter wave length phase shifter positioned in said circular wave guide arm intermediate said junction and said resonator, whereby the energy incident in one of said rectangular wave guide branches is divided between two opposite rectangular wave guide branches adjacent said one wave guide branch as a function of said frequency difference.

6. Apparatus for frequency discrimination at high frequencies comprising, a hollow wave guide junction formed by a circular wave guide arm and four rectangular wave guide branches, said rectangular wave guide branches being parallel connected and symmetrically joined with said circular wave guide arm, the axes of said five wave guide branches meeting in a common point and being mutually perpendicular, a tunable cavity resonator coupled to the extremity of said circular wave guide arm, the proportion of energy reflected from said resonator being proportional to the difference between the frequency of energy in said arm and the resonant frequency of said resonator, and a plurality of pins positioned in the wall of said circular wave guide arm to form an effective quarter wave length plate having its plane of symmetry disposed at an angle of 45 with the plane containing opposite arms of said rectangular wave guide junction and the axis of said circular wave guide arm, whereby the energy incident in one of said rectangular wave guide branches is divided between two opposite rectangular wave guide branches adjacent said one wave guide branch as a function of said frequency difference.

7. Apparatus for frequency discrimination at high frequencies comprising, a hollow wave guide junction formed by a circular wave guide arm and four rectangular wave guide branches, said rectangular wave guide branches being parallel connected and symmetrically joined with said circular wave guide arm, the axes of said five wave guide branches meeting in a common point and being mutually perpendicular, a cavity resonator coupled to the extremity of said circular wave guide arm, the proportion of energy reflected from said resonator being proportional to the difference between the frequency of energy in said arm and the resonant frequency of said resonator, and a plurality of pins positioned in the wall of said circular wave guide arm to form a quarter wave length plate in effect having its plane of symmetry disposed at an angle of 45 with the plane containing opposite arms of said rectangular wave guide junction and the axis of said circular wave guide arm, said resonator being tunable over a band of frequencies whereby energy at a fixed frequency incident to one of said rectangular wave guide branches is divided between opposite rectangular wave guide branches adjacent said one wave guide branch so that the ratio of divided power is a function of the difierence of the resonant frequency of said resonator to the frequency of the incident energy.

8. Apparatus for frequency discrimination at high frequencies comprising, a hollow wave guide junction formed by a circular wave guide arm and four rectangular wave guide branches, said rectangular wave guide branches being parallel connected and symmetrically joined with said circular wave guide arm, the axes of said five wave guide branches meeting in a common point and being mutually perpendicular, a plate positioned transverse to the axis of said circular wave guide arm and having a coupling aperture to form with said circular wave guide a cavity resonator for the reflection of energy in said circular wave guide arm in proportion to the difference between the frequency of energy in said arm and the resonant frequency of said cavity resonator, a shorting plunger adjustably mounted in the extremity of said circular wave guide arm remote from said junction for changing the resonant frequency of said resonator, and a quarter wave length phase shifter positioned in said circular wave guide arm between said junction and said resonator, whereby the energy incident in one of said rectangular wave guide branches is divided between two opposite rectangular wave guide branches adjacent said one wave guide branch as a function of said frequency difference.

9. Apparatus as in claim 5 and including detecting means in said opposite two rectangular wave guide branches, the difference voltage output of said detectors being a function of the frequency of said incident energy related to the resonant frequency of said cavity.

l0. Apparatus as in claim 6 and including detecting means in said opposite two rectangular wave guide branches, the difference voltage output of said detectors being a function of the frequency of said incident energy related to the resonant frequency of said cavity.

11. Apparatus as in claim 7 and including detecting means in said opposite two rectangular wave guide branches, the difference voltage output of said detectors being a function of the frequency of said incident energy related to the resonant frequency of said cavity.

12. Apparatus as in claim 8 and including detecting means in said opposite two rectangular wave guide branches, the difference voltage output of said detectors being a function of the frequency of said incident energy related to the resonant frequency of said cavity.

References Cited in the file of this patent UNITED STATES PATENTS 2,410,840 Samuel Nov. 12, 1946 

